Post-fire changes in tree diversity, composition and carbon in seasonal forests in the Southern Amazonia

Fire is a major driver of forest degradation and biodiversity loss in the Amazon. Although some studies have evaluated the impacts of fire on forest structure and diversity, uncertainties remain, particularly regarding how increased fire frequency affects species composition and carbon dynamics in seasonal forests of the southern Amazon. Here, we tested two hypotheses: (1) increased fire frequency alters floristic composition and reduces alpha diversity, number of tree species, stem density, and carbon stocks; and (2) after fire, tree species that prefer forest habitats decline, while cerrado and generalist species increase. We surveyed all trees with a diameter greater than 15 cm across 14 plots (10 x100 m) in single-burned, multiple-burned and unburned primary forests. Our results showed significant shifts in species composition, as well as reductions in aboveground carbon stocks and stem density following fires, which demonstrates the sensitivity of these forests to fire events. In contrast, the lack of variation in the number of tree species and alpha diversity among forest classes suggests that these parameters are not reliable predictors of the effects of fire. Increased fire frequency led to a decline in forestpreferring species, while the numbers of cerrado and generalist species remained stable. Our findings indicate that frequent fires are reshaping forest structure and homogenizing species composition, reducing typical forest species while favoring open-area and generalist species in burned forests. This provides evidence of a potential adaptation in species composition, indicating a process of biotic homogenization driven by forest degradation following single or multiple burns.